Chromium-iron alloy



United States Patent 3,269,833 CHROMIUlVI-IRON ALLOY David M. Scruggs, Southfield, Mich., assignor to The Bendix Corporation, a corporation of Delaware Filed Mar. 26, 1965, Ser. No. 442,993 Claims. (Cl. 75-126) The present invention is a continuation-in-part of my co-pending commonly-assigned patent application Ser. No. 182,074, filed March 23, 1962, now U.S. Patent No. 3,175,279, entitled Ductile Chromium Com-position, which, in turn, is a continuation-in-part of my prior application Serial No. 88,302, filed Feb. 10, 1961, now abandoned.

The present invention relates to a new and improved alloy of chromium and iron having improved malleability and ductility, particularly at low or room temperatures compared to alloys of present commercial grades.

In the iron-chromium alloy system, alloys containing more than 25 to 26% chromium are virtually unused in the present state of the art since they have heretofore been extremely brittle and non-workable at normal temperatures, thus seriously limiting the use of high chromium alloys.

I have discovered that when spinels or spinel-like oxide formations in finally-divided form are distributed throughout a chromium rich iron alloy and said alloy is held at elevated temperatures for a period of time, the spinel acts as repositories for minute amounts of embrittling impurities such as nitrogen, oxygen, carbon, and sulphur and thereby purify the alloy, increasing the room temperature and elevated temperature malleability to a degree not heretofore obtainable, except by prohibitively expensive laboratory techniques.

Spinels are -a mixture of metal monoxide and metal sesquioxide which combine according to the general formula MO-M O (or M-M O where the Ms may be the same or dissimilar metals of the appropriate valence. Monoxide and sesquioxide unite in a complex cubic crystalline structure; however, the cubic crystalline form is not critical to the practice of the present invention and certain metal oxides which combine according to the spinel formula but have tetragonal symmetry or distorted cubic symmetry rather than a perfect cubic symmetry can be used. The term spinel-like oxide used herein refers to the broader category including true spinels, inverse spinels, and those compositions corresponding to the spinel formula but having a distorted type of structure, usually tetragonal.

The true spinel structure is essentially a close packed face centered cubic array of oxygen ions with divalent cations in A5 of the tetrahedral vacancy positions and trivalent cations in /2 the octahedral vacany positions. Inverse spinels exchange positions of the divalent ions with the trivalent ions. The same coordination may be held while distorted by larger divalent ions as in calcium chromate, CaCr O forming a distorted quasi-spinel structure which no longer has cubic symmetry but is tetragonal and remains functional as an electronic isomorph.

It is an object of my invention to provide a chromium iron alloy having a high chromium content and which includes a small percentage of spinel or spinel-like metal oxide distributed throughout to significantly increase low and elevated temperature malleability of the alloy body even though the alloy constituents are formed of relatively impure commercial grade materials which are readily available.

Other objects and advantages of the present invention will become apparent with reference to the accompanying description and drawings.

The several test and control examples described herein were uniformly processed and tested as described below.

Alloys containing various quantities of chromium and iron, both with and Without spinel for comparison purposes, were fabricated into pellets /s" thick by /2 diameter by die pressing minus 325 mesh powder at 20,000 p.s.i. These pellets were then sintered in a dry hydrogen, palladium purified atmosphere in a silicon carbide resistor muflle furnace at the temperatures listed in accompanying tables for one hour. The pellets were then ground to a flat finish on one side with emery paper and tested for hardness and malleability.

The hardness test consisted of placing the pellet in a conventional hardness test machine with a ten millimeter Brinell indenting ball in place. The load was applied in increments and the diameter of the impression made by the indenter at each increment was measured and recorded. The load was then increased to 10,000 lbs. or until the pellets cracked. The volume of metal displaced by the indenter is a function of the malleability or ductility of the material; thus, by geometrical considerations the fourth power of the indentation diameter is a deformation number which is directly related to the volume of metal displaced. This can be used to compare the amount of metal displaced at a given load, which, in turn, is related to the malleability or deformability of the particular specimen. The indentation test, briefly described above, was developed by W. H. Lenz, associated with the Los Alamos Scientific Laboratories and is more fully set forth in Los Alamos Scientific Laboratory Report LAMS 2906 entitled, A New Brittleness Test for Powder Metallurgy Materials, which can be obtained through the Office of Technical Services, United States Department of Commerce.

In Table I below, there are listed eight examples of spinel containing chromium-iron alloys in accordance with the teachings of the present invention fabricated and tested in the above-described manner. A uniform proportion of spinel relative to chromium was maintained in Example 1 through 8 of 6% by weight spinel relative to the chormium content.

3 In the above table, :1 represents the fourth power of the diameter of the impression made by the Brinell indenter at the load indicated in the adjacent column and is proportional to malleability.

For comparison, eight similar pellets were prepared of Generally, I have found that chromate and aluminate spinel-like oxides are the most effective and provide the least deleterious effect on the high temperature capability of the metal alloy. The spinels may either be added direct as spinel powder or in the case of chromate spinels, a

chromium iron alloys excluding the spinel and fabricated 5 metal oxide formed from a metal in Group II of the and tested identically with Examples 1 through 8, giving atomic table may be added and will form a chromate the malleability or deformation data recorded in Table spinel during the sintering process with a somewhat im- II below. pure commercial grade chromium metal as more fully TABLE 11 Iron (wt. Chromium Sintering Load on Example percent) (wt. percent) Temperature d Indenter 0. (lbs.)

The data recorded in Tables I and II are plotted on the described in my prior application Serial Number 182,074, graph of FIGURE 1. The graph isaplot of malleability 25 now Patent No. 3,175,279. From a cost standpoint, (d versus chromium or combined chromiumspinel conchemical inertness, high temperature capability and overtent of the chromium-iron alloy. Curve 1 is a plot of all general applicability, the spinel-like oxides or mixtures spinel containing Examples 1 through 4, whereas curve 2 thereof listed below are preferred: is a plot of the control Examples 9 through 12. Since mu II Cr Grou ,1 me a acceptable ductility or malleability can be obtained with 30 g g OX1 203 p I t 10 lde Chmmate conventional non-spinel contaimng chromium-iron alloys cacrzo4 calcium chmmate (distorted spinel) having chromium contents of less than 25%, the curves MgAl O magnes1um alummate have not been extended below 25 chromium. At

ZnAl O z1nc alummate slightly greater than d0% chrom1um content, a sigma Mgcr2o magnesium chmmate phase 15 produced wh1ch 1s unusable because of lnter- NiA12O4 nicke1 aluminate metallic formation of the constituents causes the alloy to Nicr2O4 niCke1 chmmate be very bnttle and hard.

The right-hand portion of FIGURE 1 is a lot ab It can be apprec1ated that spmels are readily ava1lable, low the sigma phase region wherein curve 3 is a plot of spinel 9 the percentagis employed, f mlmmum containing Examples 5 through 3 and Curve 4 of the 00m 40 dllution of the alloy matenal. This invention provides a "01 Examples 13 through It will be Observed that all low cost practical means to achieve ductility in chromiumspinel containing chromium-iron alloys have increased Iron anoyf not alfallable, m Wrought or cast formmalleability compared to the controls with a general trend Theddespnbed g readlly.a(.iaptable for large Scale indicated of a greater proportionate increase with higher g g f fi S fi li g i l fa g properi chromium content. This trend will continue to the 100% fg g s an ma ea 1 I y or m 1 I y o t e ase meta i t on as a ii f Other metals and additives may be added to the chromfid g 212 13 2 Y' erencfi Pa i s a ma 10H ena ium-iron alloy described herein for special purposes as um 1 7 P Pawn/{NO- 7 7 desired and the malleability improvement described herein The effect of dlffefent spinel Proportlons felatlfe to will be obtained as long as the spinel containing chromithe chromium content 1s recorded in Table III wherein the umdron ll i h continuous phase f the end material test pellets were fabricated and tested 1n the previouslyand such additives are in solution or form a discontinuous described manner. phase.

TABLE III Spinel (wt. Cr-Spinel Sintering Load on Example Iron (wt. percent comcombined Temp, d Indenter,

percent) pared to Cr) wt. percent 0. lbs.

Examples 17 through 21 are plotted on the curve of FIG- I claim: URE 2 which plots spinel content versus malleability. 1. A chromium-iron alloy having a continuous phase As can be observed, malleability increases with spinel consisting of chromium, iron, and a spinel-like oxide discontent up to a peak at 6 to 7.5% by weight. A desirable persed therein; said spinel-like oxide comprising less than malleability improvement is obtained up to approxiabout 10% by weight relative to the chromium content; mately 10% by weight sp1nel. Beyond this point the disand the combined chromium and spinel-like oxide content persion hardening effect of the ceramic spinel becomes comprising greater than 25% by weight of the continuous a negatlve factor, reducing the effectiveness of further addiphase. tions. Thus, compared to the chromium content, a maxi- 2. A chromium-iron alloy having a continuous phase mum of 10% spinel is recommended with a preferred persed of chromium, iron, and a spinel-like oxide disrange from 1.5 to 7.5 persed therein; said spinel-like oxide comprisng from 1.5%

to 7.5% by weight relative to the chromium content; and the combined chromium and spinel-like oxide content comprising greater than 25% by weight of the continuous phase.

3. A chromium-iron alloy having a continuous phase persed of chromium, iron, and a spinel-like oxide dispersed therein; said spinel-like oxide comprising from 1.5% to 7.5 by weight relative to the chromium content; and the combined chromium and spinel-like oxide content comprising from 25% to 40% by weight of the continuous phase.

4. A chromium-iron alloy having a continuous phase consisting of chromium, iron, and a spinel-like oxide dispersing therein; said spinel-like oxide comprising from 1.5% to 7.5% by weight relative to the chromium content; and the combined chromium and spinel-like oxide content comprising greater than 60% by weight of the continuous phase.

5. A chromium-iron all-0y having a continuous phase consisting of chromium, iron, and a spinel-like oxide dispersed therein; said spinel-like oxide selected from the groups consisting of chromate spinels, aluminate spinels,

6 and mixtures of chromate and aluminate spinels; said spinel-like oxide comprising from 1.5 to 7.5 by weight relative to the chromium content; and the combined chromium and spinel-like oxide content comprising greater than by weight of the continuous phase.

References Cited by the Examiner UNITED STATES PATENTS 2,588,700 7/ 1945 Carpenter -126 X 2,972,529 2/ 1961 Alexander et a1. 75206 X 3,019,103 1/1962 Alexander et a1 75206 3,150,966 9/ 1964-Rossborough 7558 X 3,152,389 10/1964 Alexander et a1 29-182.5 3,159,908 12/1964 Anders 75--206 X 

1. A CHROMIUM-IRON ALLOY HAVING A CONTINUOUS PHASE CONSISTING OF CHROMIUM, IRON AND A SPINEL-LIKE OXIDE DISPERSED THEREIN; SAID SPINEL-LIKE OXIDE COMPRISING LESS THAN ABOUT 10% BY WEIGHT RELATIVE TO THE CHROMIUM CONTENT; AND THE COMBINED CHROMIUM AN SPINEL-LIKE OXIDE CONTENT COMPRISING GREATER THAN 25% BY WEIGHT OF THE CONTINUOUS PHASE. 